1. Field of the Invention
The present invention relates to a magnetic field detecting element, and more particularly to the element structure of a magnetic field detecting element having a pair of free layers.
2. Description of the Related Art
As a reproduction element of a thin film magnetic head, GMR (Giant Magneto Resistance) elements are known. Hitherto, CIP (Current In Plane)-GMR element, in which sense current flows in a direction that is horizontal to the film surface of the element, have been mainly used. In recent years, however, in order to cope with higher recording density, elements have been developed in which sense current flows in a direction that is perpendicular to the film surface of the element. TMR elements utilizing the TMR (Tunnel Magneto-Resistance) effect, and CPP (Current Perpendicular to the Plane) elements utilizing the GMR effect are known as the elements of this type. In this specification, an element in which sense current flows in a direction that is perpendicular to the film surface of the element is generally referred to as a CPP-type element.
Conventionally, the CPP element includes a stack having a magnetic layer (free layer) whose magnetization direction changes in accordance with an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is fixed with respect to the external magnetic field, and a non-magnetic intermediate layer sandwiched between the pinned layer and the free layer. On both sides of the stack with regard to the track width direction, bias magnetic layers for applying a bias magnetic field to the free layer are provided. The free layer is magnetized into a single magnetic state by a bias magnetic field emitted from the bias magnetic layers. This provides an improvement in linearity of a change in resistance in accordance with a change in an external magnetic field, and an effective reduction in Barkhausen noise. A relative angle between the magnetization direction of the free layer and the magnetization direction of the pinned layer changes in accordance with an external magnetic field, and as a result, electric resistance of sense current that flows in a direction perpendicular to the film surface of the stack is changed. By making use of this property, external magnetization is detected. The stack is magnetically shielded by shield layers on both sides thereof with regard to the direction of stacking.
In recent years, higher track recording density is desired. However, an improvement in track recording density requires a reduction in the spacing between upper and lower shield layers (a gap between shields). In order to achieve this, a decrease in thickness of the stack is required. However, there is a large limitation that originates from the layer configuration in the conventional CPP-type elements. Specifically, since the pinned layer requires that the magnetization direction be firmly fixed without being influenced by an external magnetic field, a so-called synthetic pinned layer is usually used. The synthetic pinned layer includes an outer pinned layer, an inner pinned layer, and a non-magnetic intermediate layer which consists of Ru or Rh and which is sandwiched between the outer pinned layer and the inner pinned layer. Moreover, an antiferromagnetic layer is provided in contact with the outer pinned layer in order to fix the magnetization direction of the outer pinned layer. The antiferromagnetic layer typically consists of IrMn. In the synthetic pinned layer, the antiferromagnetic layer is coupled to the outer pinned layer via exchange-coupling so that the magnetization direction of the outer pinned layer is fixed. The inner pinned layer is antiferromagnetically coupled to the outer pinned layer via the non-magnetic intermediate layer so that the magnetization direction of the inner pinned layer is fixed. Since the magnetization directions of the inner pinned layer and the outer pinned layer are anti-parallel to each other, magnetization of the pinned layer is limited as a whole. Despite such a merit of the synthetic pinned layer, however, a large number of layers are required to constitute a CPP-type element that includes the synthetic pinned layer. This imposes limitation on a reduction in the thickness of the stack.
Meanwhile, a novel layer configuration that is entirely different from that of the above-mentioned conventional stack has been proposed in recent years. In U.S. Pat. No. 7,019,371, a stack used for the CIP element, which includes two free layers and a non-magnetic intermediate layer that is sandwiched between the free layers, is disclosed. In U.S. Pat. No. 7,035,062, a stack used for the CPP-type element, which includes two free layers and a non-magnetic intermediate layer that is sandwiched between the free layers, is disclosed. In these elements, two free layers are exchange-coupled via a non-magnetic intermediate layer due to the RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction. A bias magnetic layer is provided on the side of the stack that is opposite to the air bearing surface, and a bias magnetic field is applied in a direction that is perpendicular to the air bearing surface. The magnetization directions of the two free layers adopt a certain relative angle because of the magnetic field applied from the bias magnetic layer. If an external magnetic field is applied from a recording medium in this state, then the magnetization directions of the two free layers are changed. As a result, the relative angle between the magnetization directions of the two free layers is changed, and accordingly, electric resistance of sense current is changed. By making use of such a property, it becomes possible to detect an external magnetic field. Such a layer configuration using two free layers has the potential for facilitating a reduction in the gap between the shield layers, because it does not require a conventional synthetic pinned layer and a antiferromagnetic layer and allows a simplified layer configuration.
In such an element that uses two free layers, the requirement is that the non-magnetic intermediate layer not only produces magnetoresistance effect, but also causes the two free layers to be coupled in an anti-parallel manner by the RKKY interaction. As a material to satisfy such a requirement, a metallic material, such as Cu, can be preferably used.
However, if a metallic material, such as Cu, is used, then a large amount of sense current flows in the stack because of small electric resistance of the non-magnetic intermediate layer. This causes the problem in which it is difficult for the relative angle between free layers to be changed by an external magnetic field due to the spin-torque effect. The spin-torque effect refers to the phenomenon that spin-polarized electrons are injected into the free layer so that the magnetization state of the free layer is disturbed. This phenomenon leads to deterioration in response of an element to an external magnetic field. Since the spin-torque effect becomes more pronounced in accordance with an increase in the density of sense current, it is necessary to limit the spin-torque effect by using a semiconductor material, such as MgO, ZnO, or an insulating material, such as AlO, as the non-magnetic intermediate layer, in order to lower current density. However, these materials do not necessarily have the property to produce a RKKY interaction. Moreover, even if these materials have the property, it is necessary for the non-magnetic intermediate layer to have a specific thickness to produce the RKKY interaction. However, a sufficient magnetoresistance effect is not necessarily obtained with a specific thickness. As an example, it is reported that when MgO is used as the non-magnetic intermediate layer, weak RKKY interaction (exchange-coupling constant 2.6×10−12 J/m2) is obtained with a thickness of 0.6 nm. However, this thickness does not provide a magnetoresistance ratio having a practical level. Thus, in the CPP-type element using two free layers, there are large limitations on the selection of material and thickness of the non-magnetic intermediate layer, leading to a difficulty in obtaining a sufficient magnetoresistance ratio while limiting the spin-torque effect.